1. Field of the Invention
The present invention relates to a target supply unit to be used for supplying a target in an extreme ultraviolet (EUV) light source apparatus and a method of manufacturing the target supply unit.
2. Description of a Related Art
In recent years, as semiconductor processes become finer, photolithography has been making rapid progress toward finer fabrication. In the next generation, microfabrication at 60 nm to 45 nm, further, microfabrication at 32 nm and beyond will be required. Accordingly, in order to fulfill the requirement for microfabrication at 32 nm and beyond, for example, exposure equipment is expected to be developed by combining an EUV light source for generating EUV light having a wavelength of about 13 nm and reduced projection reflective optics.
As the EUV light source, there is an LPP (laser produced plasma) type EUV light source using plasma generated by applying a laser beam to a target. The LPP type EUV light source has advantages that extremely high intensity close to black body radiation can be obtained because plasma density can be considerably made higher, that light of only the particular waveband can be radiated by selecting the target material, and that an extremely large collection solid angle can be ensured because it is a point source having a substantially isotropic angle distribution and there is no structure such as electrodes surrounding the light source. Therefore, the LPP type EUV light source is predominant as a light source for photolithography.
In the LPP type EUV light source apparatus, the EUV light, which is emitted from plasma generated by applying a laser beam to a target material of tin or the like within a vacuum chamber, is reflected by an EUV collector mirror provided within the vacuum chamber and emitted to the outside. When debris generated from the target material adheres to the EUV collector mirror, the reflectivity of the EUV collector mirror for EUV light having a wavelength of 13.5 nm becomes lower, and as a result, EUV light output emitted to the outside becomes lower. On this account, it is necessary to reduce the debris generated from the target material.
As a related technology, Japanese Patent Application Publication JP-P2008-98081A discloses a method of adjusting energy of a laser beam to suppress generation of debris. Further, there is known a method of suppressing debris by supplying the minimum amount of target necessary for obtaining desired EUV energy. According to this method, typically, the target is formed in minute spherical shapes having diameters of several micrometers to several tens of micrometers. In order to obtain the shapes, a molten metal is injected from a microscopic injection hole having a diameter of several tens of micrometers formed in a target nozzle into vacuum. However, since the injection hole is extremely narrow, there has been a problem that oxides contained in the molten metal, impurities transferred from a target container or contained in the molten metal, a metal solidified due to temperature nonuniformity, or the like clogs the injection hole, and injection of the molten metal becomes impossible. Particularly, the most common cause of the injection hole clogging is oxides adhered to a metal surface before melting or contained in the metal, or oxides adhered to an inner wall of the target container.
The applicant have searched, but not found any prior art documents that point out problems about clogging in the target injection hole of the target supply unit or disclose their technical solutions.